Among piston rings used in an internal combustion engine, a top ring closest to a combustion chamber vigorously collides, due to combustion pressure, with a piston ring groove (a ring groove) of a piston made of aluminum alloy or the like and, simultaneously, slides on a surface of the ring groove. Combustion of fuel causes high temperature inside the internal combustion engine, and it is known that, in the vicinity of the top ring of a gasoline engine, the temperature rises to 200° C. or higher, causing a decrease in strength of the piston due to a thermal shock and the like.
On the surface of the piston ring groove, as illustrated in FIG. 1, projections of approximately 1 μm in height are formed at 0.2 mm intervals by a turning process of a tool. These projections are lost or worn away due to the colliding and sliding of the piston ring, exposing a new aluminum surface of the ring groove. The new aluminum surface easily adheres to the surface of the piston ring made of metal, which phenomenon will be referred to as “aluminum cohesion” hereinafter. The aluminum cohesion promotes abrasion of the ring groove. When the abrasion of the ring groove advances, sealing property of the piston ring becomes deteriorated, increasing blowby in which a combustion gas flows into a crank chamber from the combustion chamber. An increase in a blowby gas may possibly cause troubles including a reduction in power of the engine.
In order to prevent the aluminum cohesion, there have been suggested a method of avoiding direct contact between the ring groove and the piston ring and a method of buffering a damaging force applied to the ring groove by the piston ring.
As a countermeasure taken for the piston, as disclosed in PLT 1 set forth below, there has been suggested a method of carrying out anodic oxidation treatment (anodizing treatment) on the ring groove and, further, filling a lubricant material in micropores generated by the anodic oxidation treatment of the ring groove. Since a hard oxide coating is formed on the surface of the ring groove by the anodic oxidation treatment, loss of aluminum is prevented and the aluminum cohesion becomes less likely to occur. However, there are issues that the anodic oxidation treatment of the piston is costly and, due to the hardness of the oxide coating, initial compatibility is low.
As a countermeasure taken for the piston ring, as disclosed in PLT 2 set forth below, there has been suggested a method of forming, on an upper surface and a lower surface of the piston ring, a coating made of polyamide and the like, which is a heat resistant resin, containing molybdenum disulfide or the like serving as a solid lubricant dispersed therein, thereby alleviating the damaging force applied to the ring groove.
PLT 3 set forth below suggests a method of forming a coating layer containing polyamide imide as a main component, a polyamide imide coating layer modifier, and a dry coating lubricant containing hard particles such as alumina and the like on a sliding surface of a sliding member having abrasion of predetermined surface roughness, thereby improving abrasion resistance and tight-contact of the sliding member while reducing a friction coefficient.
PLT 4 set forth below suggests a method of laminating, on the upper surface and the lower surface of the piston ring, an outermost surface layer made of the heat resistant resin containing metal powder and a base layer made of the heat resistant resin, thereby improving the abrasion resistance and the tight-contact.
Recently, an increase in power of the engines have been promoted from the viewpoint of a measure for addressing environmental issues, leading to a further increase in the temperature in the vicinity of the top ring. Such circumstances have been making it difficult to maintain the resin coating on the piston ring over a prolonged period of time and sustain an effect of preventing the aluminum cohesion.
PLT 5 set forth below discloses a cylinder liner having a plateau structure obtained by processing a surface of a coating formed on an inner circumferential surface of the cylinder liner on which the piston ring slides, and a method of the processing.